In certain types of atomic frequency standards and magnetometers, light is generated and used, for example, for optical pumping and for detecting an atomic (e.g., clock) transition. In such atomic frequency standards and magnetometers, the stability of the light intensity over long periods of time (many years) is desired in order to maintain the integrity of the standard or magnetometer.
An atomic frequency standard is a device that uses an atomic transition frequency derived from an atomic or molecular species experiencing a transition between two or more well-defined energy levels of the atom or molecule to control a standard frequency oscillator.
For example, the two lowest energy levels of the rubidium atom (Rb) are known as the ground state hyperfine energy levels A and B. When atoms of gaseous Rb-87 are interrogated (irradiated) with microwave energy at a precise "transition frequency", corresponding to the rubidium frequency that causes atom transitions between the hyperfine energy levels A and B, the rubidium atoms at hyperfine energy level A will make the transition to level B, and vice versa. The transition is employed as a highly accurate frequency reference to which the frequency of a quartz crystal oscillator or voltage-controlled crystal oscillator (VCXO) can be electronically locked in creating an atomic frequency standard.
For example, in atomic frequency standards the frequency of a controllable frequency source, such as a quartz crystal oscillator, is controlled by means of a physics package and associated electronics that are devoted to maintaining the assigned output frequency, typically 5 MHz or 10 MHz, on a very long-term, exceedingly accurate and stable basis. By properly slaving the quartz crystal oscillator to the frequency of the atomic transition in the physics package, the tendency of the quartz crystal to exhibit drifting due to aging and other inherent as well as environmental effects is markedly suppressed. The physics package of a typical, passive, gas-cell, atomic frequency standard generally includes a microwave cavity resonator, an isotopic filter cell, an absorption cell, a light source, a photodetector, temperature control means, at least one magnetic shield surrounding these components, and a C-field coil.
In a typical rubidium atomic frequency standard, the light source 11 is a glass bulb containing rubidium atoms which produces light by an rf-excited plasma discharge. The rubidium in the lamp is heated to a vapor state, approximately 110.degree. C., and is subjected to a high-energy rf field from an exciter coil surrounding the glass bulb, thereby generating light from the excited rubidium atoms. As shown diagrammatically in FIG. 1, the "rubidium light" is directed through a filter cell 25a which contains an isotope of rubidium, such as Rb-85, which filters out light with a wavelength that will stimulate transition of atoms from the hyperfine energy level B to any optically-excited level C. The filtered rubidium light is then directed through an absorption cell 25, also called a resonance cell. The absorption cell 25 includes another isotope of rubidium, Rb-87, and the filtered light energy absorbed by the Rb-87 atoms at hyperfine energy level A causes a transition of the Rb-87 atoms from level A to any optically-excited energy level C. The atoms excited to energy level C, however, do not remain at level C for more than tens of nanoseconds, but return to ground state hyperfine levels A and B in approximately equal numbers by either spontaneous emission of light and/or by collisions, including collisions with other atoms, molecules, or the walls of the absorption cell 25. Since the filtered light does not allow transitions of atoms from level B to level C, the continuing cycle of optical excitation of atoms from level A to level C and the redistribution of atoms falling from level C between levels A and B eventually results in few, if any, atoms at level A for excitation to level C, and little or no absorption of the light passing through the absorption cell 25 because the atoms have accumulated at hyperfine energy level B. The atoms at level A are said to have been "optically pumped" to level B. If, however, microwave energy is applied to the absorption cell 25 at the rubidium transition frequency, transitions of atoms between hyperfine levels A and B occur, re-introducing atoms at level A which again absorb light energy and undergo a subsequent transition to level C and thereby reduce the light passing through the absorption cell 25.
The rubidium light passing through the absorption cell 25 is incident on a photodetector 16, which produces a current output which is proportional to the intensity of the incident light. In a frequency-locked loop, current output is processed by servo electronics to provide a control voltage to a voltage controlled crystal oscillator (VCXO) whose output is multiplied (and synthesized) to the rubidium transition frequency and provides the microwave energy used to cause the transitions between hyperfine levels A and B. When the frequency of the microwave energy corresponds to the hyperfine transition frequency, about 6.834 GHz for Rb-87, maximum light absorption occurs and the current output of the photodetector 16 is reduced. If, however, the frequency of the microwave energy does not correspond to the hyperfine frequency, then more light will pass through the absorption cell 25 to the photodetector 16, which in turn increases its current output. Thus, the photodetector current output can be used to provide an error signal to maintain the output frequency of the VCXO, typically 5 or 10 MHz, (which, as noted above, is multiplied and synthesized to produce the hyperfine transition frequency of the rubidium atoms), thereby creating an extremely stable 5 or 10 MHz output frequency standard.
For years, the inventors have known that the intensity of the light reaching the photodetector decays slowly over time, and thereby degrades the performance of the frequency standard. For example, as previously discussed, in a typical frequency standard, the light is eventually detected by a photodetector, which produces a photocurrent that is proportional to the intensity of the incident light. Since an ac detection scheme is used in these devices, the signal information appears as a very small modulated (ac) component (fundamental) of the total photocurrent which has the property that it is zero when the dc photocurrent is a minimum. The modulated photocurrent is processed electronically by a frequency-locked loop to produce an error signal which is used to steer the VCXO, keeping it on frequency: when the VCXO is on frequency the light reaching the photodetector is a minimum and in this case the ac component (fundamental) of the photocurrent disappears so that there is no error signal. If the VCXO frequency drifts, then the photocurrent is no longer a minimum and an ac signal and associated error signal results.
For example, if the VCXO frequency drifts away from the nominal stabilized value by a certain amount, this will produce an ac signal. The larger this signal is, the larger the frequency-locked loop gain and the better the loop can lock the VCXO frequency to the hyperfine frequency. Generally, the greater the intensity of the rubidium light, the larger is this ac signal. Thus, it is advantageous to have high light intensity. Since the photocurrent is electronically converted to a proportional voltage (the "light voltage"), a high light voltage is likewise advantageous; any decrease in the light voltage (corresponding to a decrease in light intensity) tends to reduce the gain of the frequency-locked loop and worsens the standard's frequency stability.
The intensity of the light that excites the absorption cell is also an important factor in determining the sensitivity and frequency discrimination function of the physics package. As the intensity of light at the photodetector decays, the number of atoms undergoing transition as a result of interrogation decreases with time, the light voltage from the photodetector decays, the gain of the frequency-locked loop decreases, and the performance of the frequency standard degrades. As the loop gain of the frequency-locked loop decreases further, offsets will begin to appear and the short-term frequency stability begins to worsen. Light voltage decay can also produce frequency shifts of the standard's output frequency due to the light shift effect that maps light intensity changes into frequency changes, resulting in frequency aging.
Some rubidium frequency standards are designed to increase their internal supply voltage to a high value prior to lamp ignition (this facilitates lamp ignition). After lamp ignition the supply voltage is reduced to its normal operating value by an electronic switch that is activated by the light voltage. If, for some reason, the light voltage falls below the value at which switching occurs (threshold value), the supply voltage will return to the higher value. Should the light voltage decay during normal operation so that it falls below the threshold value, the supply voltage will suddenly switch to the higher value and remain there. This will produce a major change in the unit's output frequency and render it unsuitable for its intended purpose.
Even a small decay rate in the light intensity can be a problem if it continues over a long period of time. For example, a theoretical exponential decay at a rate of 1 percent per month will cause the light voltage to decay to 50 percent of its initial value over a period of approximately six years, and a decay rate of 0.6 percent per month will cause the light voltage to decay to 50 percent of its initial value over a period of about 10 years.
The inventors have known and studied the problem of light intensity decay for years. The rate of light decay and the lower light intensity that is ultimately reached has been found to vary widely and unpredictably from apparatus to apparatus. Testing has indicated that in some groups of atomic frequency standards as little as 6 to 10 percent of the initial light intensity will ultimately be lost and in other groups of atomic frequency standards as much as 40 to 50 percent of the initial light intensity will ultimately be lost. Testing has also indicated that the rate of loss varies from group to group, with the time constant of an equivalent exponential decay being as low as 1 to 2 months with some groups and as high as 3 to 6 months in others. Many possible causes of the light decay were considered and investigated, but the primary cause of the light decay remained unknown until recently. Thus, this unpredictable loss of light intensity has continued to degrade the performance for many atomic frequency standards, and a method and apparatus that can reduce light intensity decay is needed to obtain predictable aging and substantially improve the performance of optical pumping devices, such as atomic frequency standards, magnetometers and the like.